Steel and steel component

Abstract

To provide steel excellent in surface fatigue characteristics.SOLUTION: Steel has a component composition characterized in that from the surface to the inside in order, there are a nitride compound layer and a nitrogen diffusion layer, wherein the nitrogen diffusion layer contains carbonitrides that are compatible with the base phase ferrite, the number density of carbonitrides, with a longitudinal length of 3 nm or more and 20 nm or less of the carbonitrides is at least 1.00×1023 / m3 at a position 50 μm inward from the surface, at least 0.30×1023 / m3 at a position 200 μm inward from the surface, and at least 0.10×1023 / m3 at a position 400 μm inward from the surface, the portion excluding the nitride compound layer and nitrogen diffusion layer contains, by mass percentage, C: 0.010% or more and 0.200% or less, Si: not more than 1.00%, Mn: 0.50% or more and 3.00% or less, P: not more than 0.020%, S: 0.020% or more and 0.060% or less, Cr: 0.30% or more and 3.00% or less, and V: 0.02% or more and 0.80% or less, with the remainder consisting of Fe and impurities.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to steel and steel parts, and particularly to steel and steel parts suitable for use in parts such as automobiles and construction machinery, which have a diffusion layer with a large number of fine carbonitrides formed by soft nitriding treatment and excellent surface fatigue characteristics.

Background Art

[0002] Mechanical structural parts such as automobile gears are generally required to have excellent fatigue characteristics such as surface fatigue characteristics and bending fatigue characteristics, so surface hardening treatment is usually performed. As surface hardening treatments, carburizing treatment, high-frequency quenching treatment, nitriding treatment, etc. are well known.

[0003] Among these, carburizing treatment involves infiltrating and diffusing C in the high-temperature austenite region, so a deep hardened layer can be obtained, which is effective for improving fatigue characteristics. However, since heat treatment distortion occurs due to carburizing treatment, it has been difficult to apply it to parts that require strict dimensional accuracy from the viewpoints of quietness and the like.

[0004] In addition, high-frequency quenching treatment is a process of quenching the surface layer by high-frequency induction heating, so heat treatment distortion also occurs, and there are problems in terms of dimensional accuracy similar to carburizing treatment.

[0005] On the other hand, nitriding treatment is a process of infiltrating and diffusing N in a relatively low temperature range below the Ac1 transformation point to increase the surface hardness, so the heat treatment distortion as described above is small. However, there are problems such as a long treatment time of 50 to 100 hours and the need to remove the brittle compound layer on the surface after the treatment.

[0006] Therefore, so-called soft nitriding treatment, which shortens the treatment time at a treatment temperature comparable to nitriding treatment, has been developed and has been widely popularized in recent years for parts used in machine structures and the like. This soft nitriding treatment simultaneously infiltrates N and C in the temperature range of 500 to 600 °C to form a nitrided compound layer with C dissolved in the outermost layer, and further diffuses N into the base metal to form a nitrogen diffusion layer to harden the surface layer. Compared with the conventional nitriding treatment, the treatment time can be reduced to less than half.

[0007] However, in the case of the carburizing treatment described above, it is possible to increase the core hardness by quenching and hardening, whereas the soft nitriding treatment is carried out at a temperature below the transformation point of steel, so the core hardness does not increase, and there is a problem that the soft nitrided material is inferior in fatigue characteristics compared with the carburized material.

[0008] Therefore, in order to improve the fatigue characteristics of the soft nitrided material, quenching and tempering treatment is usually carried out before the soft nitriding treatment to increase the core hardness. However, the obtained fatigue characteristics are hardly sufficient, the manufacturing cost increases, and a decrease in machinability is also inevitable.

[0009] As a solution to such problems, Patent Document 1 proposes a steel for soft nitriding that enables high bending fatigue characteristics to be obtained after soft nitriding treatment by containing Ni, Cu, Al, Cr, Ti, etc. in steel. That is, this steel is age-hardened with Ni—Al, Ni—Ti-based intermetallic compounds or Cu compounds for the core part by soft nitriding treatment, while for the surface layer part, nitrides and carbides such as Cr, Al, Ti, etc. are precipitated and hardened in the nitrided layer to improve the bending fatigue characteristics.

[0010] Further, Patent Document 2 proposes a steel for soft nitriding that can obtain excellent bending fatigue characteristics after soft nitriding treatment by hot forging and stretching a steel containing 0.5 to 2% of Cu, air-cooling it to form a ferrite-based structure with Cu dissolved, precipitating and hardening Cu during the soft nitriding treatment at 580 °C for 120 minutes, and further using the precipitation hardening of Ti, V and Nb carbonitrides in combination.

[0011] Furthermore, Patent Document 3 has proposed a steel for soft nitriding in which Ti—Mo carbides and carbides containing one or more of Nb, V, and W are further dispersed therein.

[0012] Patent Document 4 has proposed a steel for soft nitriding in which the formation of nitrides during the soft nitriding treatment is promoted by forming a dislocation structure at a high density in the steel for soft nitriding.

Prior Art Documents

Patent Documents

[0013]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0014] However, although the soft nitrided steels described in Patent Documents 1 to 3 are excellent in bending fatigue strength, the surface fatigue strength has not been considered. When sliding contact occurs in a component, if the surface fatigue strength is insufficient, cracks will occur from the contact part. The technique described in Patent Document 4 considers the hardness and carbonitrides at a depth of 50 μm from the surface layer, but does not consider the characteristics deeper inside.

[0015] The present invention significantly solves the above problems, and an object thereof is to provide a steel and a steel component that are particularly excellent in surface fatigue characteristics.

Means for Solving the Problems

[0016] In order to solve the above problems, the inventors have intensively studied the optimal microstructure in the nitrogen diffusion layer. As a result, they have found that controlling the number density and morphology of carbonitrides with a longitudinal length of 3 nm or more and 20 nm or less in the nitrogen diffusion layer is effective in improving the surface fatigue characteristics.

[0017] Also, when a steel part is subjected to sliding resistance, shear stress due to vertical resistance is applied. Since this reaches its maximum at a depth of about 400 μm from the surface, in order to exhibit excellent surface fatigue characteristics, not only the surface layer but also internal microstructure control is important.

[0018] The present invention has been completed after further study based on the above findings. That is, the gist configuration of the present invention is as follows.

[0019] 1. It has a nitride compound layer and a nitrogen diffusion layer in order from the surface to the inside, The nitrogen diffusion layer has carbonitrides that are coherent with the matrix ferrite, and the number density of carbonitrides with a longitudinal length of 3 nm or more and 20 nm or less among the carbonitrides is 1.00×10 23 / m 3 or more at a position 50 μm from the surface to the inside, 0.30×10 23 / m 3 or more at a position 200 μm from the surface to the inside, and 0.10×10 23 / m 3 or more at a position 400 μm from the surface to the inside, and The portion excluding the nitride compound layer and the nitrogen diffusion layer is, in mass%, C: 0.010% or more and 0.200% or less, Si: 1.00% or less, Mn: 0.50% or more and 3.00% or less, P: 0.020% or less, S: 0.002% or more and 0.060% or less, Cr: 0.30% or more and 3.00% or less and V: 0.02% or more and 0.80% or less and has a component composition in which the balance consists of Fe and impurities.

[0020] 2. The above composition further contains, by mass%, Nb: 0.003% or more and 0.150% or less, Al: 0.01% or more and 0.20% or less, Ti: 0.01% or more and 0.10% or less and Mo: 0.005% or more and 0.400% or less The steel according to 1 above, containing any one or two or more of these.

[0021] 3. The steel according to 1 or 2 above, wherein the carbonitride contains Cr and V as metal elements that combine with carbon and nitrogen, and the concentration of (V + Cr) in the metal elements is 50 at% or more.

[0022] 4. A steel part made of the steel according to 1, 2, or 3 above.

Advantages of the Invention

[0023] According to the present invention, it is possible to provide steel and steel parts with improved surface fatigue characteristics. Therefore, the steel and steel parts of the present invention are extremely useful as materials for mechanical structure parts such as automobiles.

Brief Description of the Drawings

[0024]

Figure 1

Figure 2

Figure 3

Modes for Carrying Out the Invention

[0025] Hereinafter, the present invention will be specifically described. As described above, in order to exhibit excellent surface fatigue characteristics in steel, not only the surface layer of the steel but also the control of the internal structure is important. In order to improve the surface fatigue characteristics of steel, it is important to precipitate fine carbonitrides at high density and even inside the steel. The presence of a large amount of carbonitrides in the surface layer inhibits the dislocation movement during surface pressure loading, making it possible to exhibit excellent surface fatigue characteristics.

[0026] That is, in steel having a nitride compound layer and a nitrogen diffusion layer in order from the surface to the inside, and the nitrogen diffusion layer having carbonitrides that match the matrix ferrite, the number density of carbonitrides having a longitudinal length of 3 nm or more and 20 nm or less among the carbonitrides is 1.00×10 23 / m 3 or more at a position 50 μm from the surface to the inside, 0.30×10 23 / m 3 or more at a position 200 μm from the surface to the inside, and 0.10×10 23 / m 3 or more at a position 400 μm from the surface to the inside. This is essential.

[0027] [Carbonitrides in the Nitrogen Diffusion Layer] First, the carbonitrides in the nitrogen diffusion layer will be described. In the present invention, the carbonitrides need to be coherent with the matrix ferrite. In the case of carbonitrides that are coherent with the matrix ferrite, the resistance to dislocations increases by generating coherent strain, making it possible to improve the fatigue characteristics.

[0028] Here, being coherent with the matrix ferrite means having a crystal orientation relationship called the Baker-nutting relationship with the matrix ferrite. In this case, (001) of ferrite and (011) of the NaCl-type carbonitride are coherent, and in order to generate sufficient coherent strain, the misfit (ε) between the two becomes important. This misfit (ε) is calculated according to the following formula (1). In the present invention, the carbonitrides that are coherent with the matrix ferrite are those in which the ε is 10% or less. Carbonitrides with ε exceeding 10% have a reduced effect of inhibiting dislocation movement. ε = |a0 - ap| / ap × 100 (%) …(1) Here, a0 represents the interplanar spacing of ferrite (= 0.2866 nm), and ap represents the interplanar spacing of carbonitrides.

[0029] The method for measuring the above interplanar spacing is not particularly limited, but it can be confirmed by directly observing the interface region using a high-resolution electron microscope observation method. For example, FIG. 1 shows a HAADF-STEM (High Angle Annular Dark-Field Scanning Transmission Electron Microscopy) image obtained by <001> incidence of carbonitrides. As shown in FIG. 1, it is possible to directly measure the interplanar spacing ap of carbonitrides from this image.

[0030] In order to improve the surface fatigue characteristics of steel, the number density of carbonitrides (hereinafter also referred to as compatible-diameter carbonitrides) having a longitudinal length of 3 nm or more and 20 nm or less among the carbonitrides that match the matrix ferrite is 1.0×10 23 / m 3 or more at a position 50 μm from the surface to the inside, 0.3×10 23 / m 3 or more at a position 200 μm from the surface to the inside, and 0.1×10 23 / m 3 or more at a position 400 μm from the surface to the inside is important. If the above conditions are not satisfied, excellent surface fatigue characteristics cannot be obtained because dislocation movement cannot be sufficiently inhibited.

[0031] Here, for carbonitrides, the reason for defining carbonitrides having a longitudinal length of 3 nm or more and 20 nm or less is that first, carbonitrides having a longitudinal length less than 3 nm are difficult to exhibit a sufficient dislocation movement inhibition effect even if they match the matrix ferrite. Also, when the length of the carbonitride exceeds 20 nm, the compatibility with the matrix ferrite deteriorates and the resistance to dislocation movement becomes small.

[0032] Furthermore, defining the number density of the above-described conforming diameter carbonitrides at positions 50 μm, 200 μm, and 400 μm from the surface inward is as described above for the 400-μm position. On the other hand, at the 50-μm position and the 200-μm position, compressive stress becomes a problem. That is, the compressive stress is maximum on the surface side and gradually decreases, but in order to have a structure that can withstand this compressive stress, it is necessary to control the structure at depth positions of 50 μm and 200 μm step by step from the surface. By defining the number density of the conforming diameter carbonitrides at the above two depth positions, it is possible to withstand the gradually changing compressive stress. And the reasons for limiting the number density of the conforming diameter carbonitrides at each position are as follows.

[0033] [At a position 50 μm from the surface inward, 1.00×10 23 / m 3 or more] Setting the number density of the conforming diameter carbonitrides at the 50-μm position to 1.00×10 23 / m 3 or more is to sufficiently ensure the resistance to the above-described dislocation movement. Although the upper limit does not particularly need to be limited, from the viewpoint of alloy cost, it is preferably 4.00×10 23 / m 3 or less.

[0034] [At a position 200 μm from the surface inward, 0.30×10 23 / m 3 or more] Setting the number density of the conforming diameter carbonitrides at the 200-μm position to 0.30×10 23 / m 3 or more is similarly to sufficiently ensure the resistance to dislocation movement. Although the upper limit does not particularly need to be limited, from the viewpoint of alloy cost, it is preferably 2.50×10 23 / m 3 or less.

[0035] [At a position 400 μm from the surface inward, 0.10×10 23 / m 3 or more] The number density of the coherent carbide nitride at the position of 400 μm is set to be 0.10×10 23 / m 3 or more, which is also to sufficiently ensure the resistance to dislocation movement. Note that the upper limit does not particularly need to be limited, but from the perspective of alloy cost, it is preferably 0.60×10 23 / m 3 or less.

[0036] The carbide nitride that matches the above-mentioned matrix ferrite precipitates on the {001} plane of the ferrite which is the matrix structure, satisfying the Baker-Nutting relationship. Therefore, the number density of the carbide nitride is calculated as follows. First, transmission electron microscope (TEM) observation was performed from the {001} plane of the matrix ferrite, and the number of carbide nitrides in the field of view was measured. Next, the relative film thickness of the sample in each observation field was measured by Electron Energy Loss Spectroscopy (EELS), and the volume of the observation field was calculated. Then, the number density was calculated by dividing the number of carbide nitrides by the volume. Here, although the carbide nitride precipitates on the (001) plane, (010) plane, and (100) plane, for example, when observed with (001) incidence, the carbide nitride on the (001) plane cannot be observed, so only two planes can be observed by TEM observation. Therefore, the number obtained by multiplying the number of carbide nitrides observed by TEM by 1.5 was used as the number of carbide nitrides. The measurement of the film thickness was taken as the value obtained by multiplying the relative film thickness measured by EELS by the mean free path λ calculated from the following calculation formula of Iakoubovskii et al. JPEG2025113135000002.jpg14170 Here, E0: acceleration voltage, α: convergence angle, β: capture angle, ρ: sample density, F=(1 + E0 / 1022) / (1 + E0 / 511) 2 、 θ E = 5.5ρ 0.3 / FE0, θ c = 20 mrad was used.

[0037] Here, in order to control the number density of the above-mentioned conforming diameter carbonitrides within the above-mentioned range at positions 50 μm, 200 μm, and 400 μm from the surface to the inside, in addition to the component composition shown below, it is preferable to perform the gradient soft nitriding treatment described later.

[0038] [Component composition of non-hardened part] Next, in the present invention, the reason for limiting the component composition of the non-hardened part excluding the nitride compound layer (hereinafter also referred to as the compound layer) and the nitrogen diffusion layer (hereinafter also referred to as the diffusion layer) to the above-mentioned range will be explained. Note that "%" representing the following component composition means "mass%" unless otherwise specified.

[0039] C: 0.010% or more and 0.200% or less C is necessary to ensure the strength of the non-hardened part, the compound layer, and the diffusion layer. Furthermore, C is necessary to form carbides in the steel material before the soft nitriding treatment. This carbide is useful for carbonitride formation during the soft nitriding treatment. When the C amount is less than 0.010%, a sufficient amount of carbonitrides is not formed after soft nitriding. Therefore, the C content is set to 0.010% or more. More preferably, the C content is 0.050% or more. On the other hand, when the C content exceeds 0.200%, the carbonitrides become coarse and the compatibility with the matrix ferrite deteriorates. Therefore, the C content is set to 0.200% or less. More preferably, it is in the range of 0.100% or less.

[0040] Si: 1.00% or less Si is effective for ensuring strength. However, when the Si content exceeds 1.00%, the workability deteriorates due to solid solution strengthening. Therefore, the Si amount is set to 1.00% or less. A more preferable Si content is 0.500% or less. From the viewpoint of ensuring the strength of the steel, the Si content is preferably 0.005% or more.

[0041] Mn: 0.50% or more and 3.00% or less Mn forms carbonitrides with N and C diffused from the surface during soft nitriding, thereby having the effect of precipitation strengthening the compound layer and the diffusion layer. When the Mn content is less than 0.50%, the amount of carbonitrides formed is insufficient, so the above-mentioned carbonitride number density decreases. Therefore, the Mn content should be 0.50% or more. Preferably, it is 1.50% or more. On the other hand, when the Mn content exceeds 3.00%, carbonitrides precipitate only in the surface layer part and cannot be precipitated to the inside. Furthermore, the (V + Cr) concentration becomes less than 50 at%, and the integrity of the carbonitrides deteriorates. Therefore, the Mn content should be 3.00% or less. Preferably, it is in the range of 2.50% or less, more preferably 2.00% or less.

[0042] P: 0.020% or less P is an element mixed into steel as an impurity and is known as a cause of surface cracking of the slab. Therefore, it is desirable to suppress the content of P as much as possible, but up to 0.020% is acceptable. Note that it requires high costs to make the P content less than 0.001%, so industrially, it is sufficient to reduce it to 0.001%.

[0043] S: 0.002% or more and 0.060% or less S is an element mixed into steel as an impurity, but on the other hand, it contributes to improving machinability. That is, when the S content is less than 0.002%, the amount of MnS formed in the steel decreases, and the machinability decreases. On the other hand, when the S content exceeds 0.060%, the amount of solid-solution Mn required during the soft nitriding treatment decreases due to the excessive precipitation as MnS, so the content is limited to 0.060% or less. Preferably, it is 0.040% or less.

[0044] Cr: 0.30% or more and 3.00% or less Cr forms nitrides and carbonitrides by diffusing N and C from the surface during soft nitriding, thereby having the effect of precipitation strengthening the compound layer and the diffusion layer. The compound layer and the diffusion layer can be formed by performing soft nitriding as described later, but by increasing the N concentration with respect to the component composition of the material before soft nitriding, they become the compound layer and the diffusion layer. In the non-hardened part, which is the part other than the compound layer and the diffusion layer, the component composition of the material before soft nitriding is maintained except for N. Therefore, although it is for the purpose of precipitation strengthening the compound layer and the diffusion layer, the Cr content is also set to 0.30% or more for the non-hardened layer. When the Cr content is less than 0.30%, the amount of CrN precipitated in the compound layer and the diffusion layer during soft nitriding is insufficient. Furthermore, the concentration of (V + Cr) in the carbonitride cannot be made 50 at% or more. Therefore, the Cr amount is set to 0.30% or more. On the other hand, when it exceeds 3.00%, carbonitrides precipitate only in the surface layer part and cannot be precipitated to the inside, so the Cr amount is set to 3.00% or less. Preferably it is 0.50% or more. Also preferably, it is 1.50% or less.

[0045] V: 0.02% or more and 0.80% or less V forms nitrides with nitrogen diffused from the surface layer during soft nitriding and contributes to the increase in the hardness of the surface layer. Also, V forms fine precipitates during soft nitriding and greatly contributes to the increase in the number density of the above-mentioned conforming diameter carbonitrides. When the V content is less than 0.02%, the amount of VN precipitated in the compound layer and the diffusion layer during soft nitriding is insufficient. Furthermore, the concentration of (V + Cr) in the carbonitride cannot be made 50 at% or more, and the integrity of the carbonitride deteriorates. Therefore, the V amount is set to 0.02% or more. On the other hand, when added in excess, carbonitrides precipitate only in the surface layer part and cannot be precipitated to the inside, so the V content is 0.80% or less, preferably 0.50% or less, more preferably 0.40% or less.

[0046] In addition to the elements described above, one or more of the following elements can be optionally contained. The optionally containable elements and their preferred contents are described below.

[0047] Nb: 0.003% or more and 0.150% or less Nb forms nitrides with nitrogen diffused from the surface layer during soft nitriding, contributing to the increase in the surface hardness. Also, Nb forms fine precipitates due to the temperature rise during soft nitriding, increasing the core hardness. Therefore, the Nb content should be 0.003% or more. On the other hand, if added in excess, carbonitrides precipitate only in the surface layer and cannot precipitate to the interior, and furthermore, the integrity of the carbonitrides deteriorates. Therefore, the Nb content is 0.150% or less, preferably 0.120% or less.

[0048] Al: 0.01% or more and 0.20% or less Al forms carbonitrides with N and C diffused from the surface during soft nitriding, thereby having the effect of precipitation strengthening the compound layer and the diffusion layer. For this purpose, it should be 0.01% or more. On the other hand, when the Al content exceeds 0.20%, carbonitrides precipitate only in the surface layer and cannot precipitate to the interior. Therefore, the Al content is 0.20% or less, preferably 0.10% or less, more preferably 0.04% or less.

[0049] Ti: 0.01% or more and 0.10% or less Ti forms carbonitrides with N and C diffused from the surface during soft nitriding, thereby having the effect of precipitation strengthening the compound layer and the diffusion layer. When the Ti content is less than 0.01%, it is difficult to ensure the strength due to insufficient carbonitrides precipitating in the compound layer and the diffusion layer during nitriding treatment. Therefore, the Ti content should be 0.01% or more. On the other hand, if it exceeds 0.10%, carbonitrides precipitate only in the surface layer and cannot precipitate to the interior, and furthermore, the integrity of the carbonitrides deteriorates. Therefore, the Ti content is 0.10% or less.

[0050] Mo: 0.005% or more and 0.400% or less Mo forms nitrides with nitrogen diffused from the surface layer during soft nitriding and contributes to the increase in surface hardness. Mo also generates bainite and contributes to the improvement of machinability and the increase in core hardness. Therefore, the Mo content should be 0.005% or more. On the other hand, if Mo is added in excess, carbonitrides precipitate only in the surface layer and cannot precipitate to the interior, and furthermore, the coherence of the carbonitrides deteriorates. Therefore, the Mo content is 0.400% or less, preferably 0.150% or less.

[0051] The component composition of the non-hardened part of the steel of the present invention is such that the remainder other than the elements described above is Fe and impurities. Impurities are those mixed in from ore, scrap, or the manufacturing environment as raw materials during the industrial production of steel, and are allowed within a range that does not adversely affect the characteristics of the present embodiment.

[0052] [Carbonitride composition] Next, the concentration of the metal elements constituting the carbonitrides will be described. When V and Cr form carbonitrides, the lattice misfit with ferrite is small. Therefore, when precipitation is carried out so that the concentration of (V + Cr) in the metal elements of the carbonitrides becomes 50 at% or more, the misfit between the carbonitrides and ferrite becomes small, and precipitation can be carried out in good coherence with ferrite. Carbonitrides with good coherence with ferrite have a large resistance to dislocation movement and can achieve excellent surface fatigue characteristics. Therefore, it is preferable that the concentration of (V + Cr) is 50 at% or more.

[0053] As metal elements constituting the carbonitrides, in addition to V and Cr, there are Mn, Nb, Al, Ti, Mo, etc., and the total concentration thereof may be 50 at% or less.

[0054] In the present invention, the concentration of each metal element constituting the carbonitrides can be measured using a three-dimensional atom probe (3DAP). That is, a region where the N concentration is 5% or more is regarded as a carbonitride, and the average concentration of 20 or more carbonitrides can be quantitatively determined.

[0055] Next, the steel structures of the compound layer and diffusion layer of the steel of the present invention will be described. The compound layer and diffusion layer are formed along with the soft nitriding treatment, and are formed by the diffusion of nitrogen and carbon in the soft nitriding treatment atmosphere into the steel. That is, on the outermost layer of the steel, an iron nitride (Fe3N or Fe4N) formed by the combination of Fe, which is the main component of the steel to be subjected to the soft nitriding treatment, and N serves as the matrix phase, and other contained components combine with nitrogen and carbon to precipitate as carbonitrides to form a compound layer. Note that the compound layer is preferably formed to have a thickness ranging from 3 to 40 μm.

[0056] Furthermore, the diffusion layer is a layer in which nitrogen diffuses into the steel and the nitrogen concentration becomes higher than that before the soft nitriding treatment, and is formed adjacent to the inside of the compound layer. Since the above non-hardened portion is a portion where no nitrogen diffusion occurs, the component composition of the non-hardened portion is as described above, while the compound layer and the diffusion layer have a component composition with a high N content compared to the component composition of the non-hardened portion. Note that the structures of the diffusion layer and the non-hardened portion inside thereof form a matrix phase of ferrite or bainite. The diffusion layer is preferably formed to have a thickness ranging from 400 to 1000 μm adjacent to the inside of the compound layer.

[0057] The steel according to the present invention has been described above. Furthermore, the steel component according to the present invention has the above-described steel of the present invention formed into the shapes of various components, for example, components for mechanical structures. Here, the steel component of the present invention is particularly preferably a toothed component such as a gear. In this case, the above-described compound layer is preferably formed at least on the surface layer portion of the tooth portion. The teeth of a toothed component such as a gear are portions where there is sliding contact and are portions where excellent surface fatigue strength is required. When the above-described compound layer and diffusion layer are formed on this tooth portion, it leads to ensuring the durability of the toothed component.

[0058] In addition, for steel parts where there are parts that experience sliding contact even if they are not toothed parts, since the surface fatigue characteristics of these parts are important for ensuring the durability of the parts, by forming the above-described diffusion layer on such parts, an effect of improving the surface fatigue characteristics can be obtained. Therefore, the steel parts of the present invention are not limited to toothed parts.

[0059] [Manufacturing Method] Next, the manufacturing method of the steel and further the steel parts of the present invention will be described. FIG. 2 shows a typical manufacturing process for manufacturing steel parts using steel (bar steel). Here, S1 is a manufacturing process for bar steel (steel for soft nitriding) as a raw material, S2 is a conveying process, and S3 is a manufacturing process for parts (soft nitrided parts including soft nitrided steel).

[0060] First, in the bar steel manufacturing process (S1), an ingot is hot-rolled and / or hot-forged into bar steel, and after quality inspection, it is shipped. Then, after conveying (S2), in the soft nitriding part finishing process (S3), the bar steel is cut into a predetermined size, hot-forged or cold-forged, and if necessary, subjected to cutting processes such as drill piercing or turning to obtain a desired shape (for example, a gear shape or a shaft shape), and then soft nitriding treatment is performed to obtain a product (steel part).

[0061] Alternatively, the hot-rolled material may be directly finished into a desired shape by cutting processes such as turning or drill piercing, and then soft nitriding treatment may be performed to obtain a product (steel part). In the case of hot forging, cold straightening may be performed after hot forging. In addition, a coating treatment such as painting or plating may be performed on the final product.

[0062] Here, for the soft nitriding treatment, it is desirable to perform soft nitriding treatment at 520 to 550°C for 2 hours or more, and then continue the soft nitriding treatment while raising the temperature at 20 to 30°C / h. After reaching 560 to 590°C, further perform soft nitriding treatment for 1 hour or more. By performing such gradient soft nitriding treatment, N and C can be diffused to a deeper region, and carbonitrides can be formed up to a deeper region.

[0063] In the soft nitriding treatment, N and C are simultaneously introduced into the steel to form a nitrided compound layer in which C is dissolved, and further N is diffused into the base metal to form a diffusion layer. Therefore, a mixed atmosphere of nitrogenous gases such as NH3 and N2 and carburizing gases such as CO2 and CO, for example, an atmosphere of NH3:N2:CO2 = 50:45:5 may be used for the soft nitriding treatment.

[0064] By performing the above soft nitriding treatment, a compound layer formed from Fe3N or Fe4N, or both, is formed on the surface layer with a thickness ranging from 3 to 40 μm. On the inner side of the compound layer, a diffusion layer in which a large number of carbonitrides are precipitated is formed with a thickness of 400 μm or more from the surface of the compound layer. Note that the diffusion layer and the structure inside the diffusion layer have a matrix phase forming a ferrite or pearlite or bainite structure.

[0065] By using the above method, it is possible to generate a structure having a desired carbonitride with a high density as described above. Note that the soft nitriding treatment is not necessarily limited to the above method, and any method may be used as long as it can form the structure of the diffusion layer defined in the present invention. By the above manufacturing process, the steel of the present invention or a component made of this steel is obtained.

Example

[0066] Hereinafter, examples of the present invention will be specifically described. Steel having the composition shown in Table 1 (Steel grades 1 to 60) was made into a slab with a cross section of 300 mm × 400 mm by a continuous casting machine. After soaking this slab at 1250°C for 30 minutes, it was hot-rolled into a steel slab with a rectangular cross section having a side of 140 mm. Further, this steel slab was hot-rolled into a round bar with a diameter of 80 mm (as a raw material as it is after hot rolling). After holding this round bar at 1200°C for 1 hour, hot forging was performed to obtain a round bar with a smaller diameter of 35 mmφ.

[0067]

Table 1

[0068] Furthermore, for the above hot-forged material (bar steel), a roller pitching test piece shown in Fig. 3 was taken parallel to the longitudinal direction (axial direction), and this test piece was subjected to soft nitriding treatment. In order to obtain a desired diffusion layer structure, the soft nitriding temperature and soft nitriding time were adjusted as shown in Table 2.

[0069]

Table 2

[0070] Regarding the obtained soft nitrided material, the measurement of the number density of carbonitrides and the evaluation of surface fatigue characteristics at 50 μm, 200 μm, and 400 μm from the surface were carried out as follows.

[0071] That is, for the measurement of the number density of carbonitrides, three fields of view were photographed at a magnification of 640,000 times using TEM and calculated according to the above-described method. Also, for the consistency, ε obtained by using the HAADF-STEM image obtained by the incidence of the {001} plane of the matrix ferrite in the above soft nitrided material and calculated according to the above-described method was determined to be consistent (〇) when it was 10% or less. When ε exceeded 10%, it was determined to be inconsistent (×).

[0072] The concentration of the metal elements constituting the carbonitrides was calculated according to the above-described method.

[0073] For the evaluation of surface fatigue characteristics, using the roller pitching test piece after soft nitriding treatment, the fatigue limit strength was measured by creating an S-N diagram with RPT-402 manufactured by Nikkokei. The fatigue limit strength was taken as the maximum stress at which the test continued for more than 10 7 cycles when N = 2 or more. Those with a fatigue limit strength of 2600 MPa or more were rated as good (〇), and those with less than 2600 MPa were rated as inferior in fatigue limit strength (×). The roller pitching test conditions were a slip ratio of 40%, using gear oil (BESCO Transaxle) as the lubricating oil, and the oil temperature was 80 °C. The rotational speed during the test was 2000 rpm. For the large roller in contact with the transfer surface, a hardened and tempered product made of SUJ2 with a crowning R150 mm was used. The above evaluation results are shown in Table 3.

[0074]

Table 3

Claims

1. having a nitride compound layer and a nitrogen diffusion layer in order from the surface to the inside, The nitrogen diffusion layer has carbonitrides that match the matrix ferrite, and the number density of carbonitrides having a longitudinal length of 3 nm or more and 20 nm or less among the carbonitrides is 1.00×10 ２３ / m ３ or more at a position 50 μm from the surface to the inside, 0.30×10 ２３ / m ３ or more at a position 200 μm from the surface to the inside, and 0.10×10 ２３ / m ３ or more at a position 400 μm from the surface to the inside, and the portion excluding the nitride compound layer and the nitrogen diffusion layer has, in mass %, C: 0.010% or more and 0.200% or less, Si: 1.00% or less, Mn: 0.50% or more and 3.00% or less, P: 0.020% or less, S: 0.002% or more and 0.060% or less, Cr: 0.30% or more and 3.00% or less and V: 0.02% or more and 0.80% or less and has a component composition in which the balance consists of Fe and impurities.

2. The component composition further has, in mass %, Nb: 0.003% or more and 0.150% or less, Al: 0.01% or more and 0.20% or less, Ti: 0.01% or more and 0.10% or less and Mo: 0.005% or more and 0.400% or less The steel according to claim 1, containing any one or two or more of these.

3. The steel according to claim 1 or 2, wherein the carbonitride contains Cr and V as metal elements that combine with carbon and nitrogen, and the concentration of (V + Cr) in the metal elements is 50 at% or more.

4. A steel part made of the steel according to claim 1 or 2.

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